1. Field of the Invention
The field of the invention is that of transmitters of signals with 2.sup.n phase states obtained by phase shift keying with M states or by amplitude and phase modulation with M states.
The invention is more precisely concerned with a quadrature modulator imbalance estimator for modulators having two mixers to which two carrier signals are applied and to each of which a modulating signal is applied, the output signals of the mixers being applied to a combiner to constitute a modulated signal. The carrier signals are in phase quadrature and are at an intermediate frequency (IF modulation) or at a carrier frequency (direct modulation).
One aim of the invention is to provide an estimator that can be used in a control loop to modify the amplitude difference of the output signals of the mixers (balance estimator adapted to estimate the relative amplitude of two channels). The invention is of particular benefit in controlling a modulator stage operating in a wide band of frequencies.
2. Description of the Prior Art
European patent application No 0 503 588 describes a phase quadrature signal modulator imbalance estimator. The disadvantage of this estimator is that it requires the use of a learning sequence for estimating the imbalance and that the correction of the modulator is not adaptive.
French patent application No 2 729 261 also describes a quadrature modulator imbalance estimator.
In the above application, the quadrature modulator includes two mixers to which two carrier signals are applied and to each of which a modulating signal is applied, the output signals of the mixers being applied to a combiner to constitute a modulated signal.
The estimator includes:
means for detecting the instantaneous power of the modulated signal; PA1 means for multiplying the detected instantaneous power by at least one of the modulating signals; and PA1 means for integrating the result of the above multiplication, the integrator means supplying a signal indicative of the modulator malfunction. PA1 means for detecting the instantaneous power of said modulated signal; PA1 means for multiplying said detected instantaneous power by said modulating signals, said multiplier means supplying produced signals; PA1 means for rectifying said produced signals; and PA1 means for integrating said rectified produced signals supplying signals indicative of said imbalance of said modulator.
The defect estimated is either a quadrature defect or carrier residuals in the modulated signal.
When the malfunction of the modulator is a defect in respect of the phase quadrature relationship of the signals from the mixers, the multiplier means multiply the detected instantaneous power by the modulating signals to supply, after integration, a signal representative of the phase difference between the signals from the mixers.
When the malfunction of the modulator is carrier signal residuals in the modulated signal, the multiplier means multiply the instantaneous power by each of the modulating signals to supply, after integration, two signals representative of the carrier signal residuals in the modulated signal.
However, the above document does not propose any solution to the problem of correcting imbalance.
FIG. 1 shows a constellation of a four-state phase shift keyed signal from a modulator having an imbalance.
In the case of modulation with 2.sup.n phase states, to obtain a Cartesian coordinate type representation in a signal space, the 2.sup.n phase states can be uniformly distributed relative to axes corresponding to orthogonal carrier signals X and Y. There is then obtained a constellation that corresponds to the vector representation of the carrier signal states whose projections onto the X and Y axes give the coordinates of the end of the vector represented.
If the modulator has an imbalance, the constellation is as shown in FIG. 1 because of the amplitude difference between the X and Y channels. This degrades transmitter performance.
One object of the present invention is to overcome the above drawbacks.
To be more precise, one object of the invention is to provide a quadrature modulator imbalance estimator that can be used to correct the imbalance in an adaptive way. In particular, it must be possible to use the estimator in a quadrature modulator operating in a wide band of frequencies.